nixos/hledger-web: add stateDir, use own user, fix ExecStart

This allows for shared hledger installations, where the web interface is
available via network and multiple user share a SSH access to the
hledger user.

Also added `--serve` to the CLI options, as hledger-web tries to open a
webbrowser otherwise:

hledger-web: xdg-open: rawSystem: runInteractiveProcess: exec: does not
exist (No such file or directory)

Co-authored-by: Aaron Andersen <aaron@fosslib.net>

nixos/modules/services/web-apps/hledger-web.nix

@@ -34,11 +34,22 @@ in {
       '';
     };
 
-    journalFile = mkOption {
+    stateDir = mkOption {
       type = types.path;
-      example = "/home/hledger/.hledger.journal";
+      default = "/var/lib/hledger-web";
       description = ''
-        Input journal file.
+        Path the service has access to. If left as the default value this
+        directory will automatically be created before the hledger-web server
+        starts, otherwise the sysadmin is responsible for ensuring the
+        directory exists with appropriate ownership and permissions.
+      '';
+    };
+
+    journalFiles = mkOption {
+      type = types.listOf types.str;
+      default = [ ".hledger.journal" ];
+      description = ''
+        Paths to journal files relative to <option>services.hledger-web.stateDir</option>.
       '';
     };
 
@@ -50,28 +61,61 @@ in {
         Base URL, when sharing over a network.
       '';
     };
+
+    extraOptions = mkOption {
+      type = types.listOf types.str;
+      default = [];
+      example = [ "--forecast" ];
+      description = ''
+        Extra command line arguments to pass to hledger-web.
+      '';
+    };
+
   };
 
   config = mkIf cfg.enable {
-    systemd.services.hledger-web = {
+
+    users.users.hledger = {
+      name = "hledger";
+      group = "hledger";
+      isSystemUser = true;
+      home = cfg.stateDir;
+      useDefaultShell = true;
+    };
+
+    users.groups.hledger = {};
+
+    systemd.services.hledger-web = let
+      serverArgs = with cfg; escapeShellArgs ([
+        "--serve"
+        "--host=${host}"
+        "--port=${toString port}"
+        "--capabilities=${capabilityString}"
+        (optionalString (cfg.baseUrl != null) "--base-url=${cfg.baseUrl}")
+        (optionalString (cfg.serveApi) "--serve-api")
+      ] ++ (map (f: "--file=${stateDir}/${f}") cfg.journalFiles)
+        ++ extraOptions);
+    in {
       description = "hledger-web - web-app for the hledger accounting tool.";
       documentation = [ https://hledger.org/hledger-web.html ];
       wantedBy = [ "multi-user.target" ];
       after = [ "networking.target" ];
-      serviceConfig = {
-        ExecStart = ''
-          ${pkgs.hledger-web}/bin/hledger-web \
-          --host=${cfg.host} \
-          --port=${toString cfg.port} \
-          --file=${cfg.journalFile}  \
-          "--capabilities=${cfg.capabilities}" \
-          ${optionalString (cfg.baseUrl != null) "--base-url=${cfg.baseUrl}"} \
-          ${optionalString (cfg.serveApi) "--serve-api"}
-        '';
-        Restart = "always";
-      };
+      serviceConfig = mkMerge [
+        {
+          ExecStart = "${pkgs.hledger-web}/bin/hledger-web ${serverArgs}";
+          Restart = "always";
+          WorkingDirectory = cfg.stateDir;
+          User = "hledger";
+          Group = "hledger";
+          PrivateTmp = true;
+        }
+        (mkIf (cfg.stateDir == "/var/lib/hledger-web") {
+          StateDirectory = "hledger-web";
+        })
+      ];
     };
+
   };
 
-  meta.maintainers = with lib.maintainers; [ marijanp ];
+  meta.maintainers = with lib.maintainers; [ marijanp erictapen ];
 }